png images not showing

I'm using Windows XP home (with SP2), and Internet Explorer 6, on my laptop, which is not connected to the internet.However, I do some work on my website on the laptop when I'm away from my home pc.Just recently, whenever I preview my site in IE, none of the png images will display-just the box with the name of the image, and a red X. The site will show properly in Firefox.Also any program that uses a cut-down version of IE to display that program's helpfile, behaves in the same way: ie: any png images in the helpfile won't display.Has my IE 6 gone wrong?